1. Field of the Invention
The present invention relates to high-frequency ICs and GPS receivers and, particularly, to a high-frequency IC suitable for a high-frequency front-end IC which converts a high-frequency signal into a baseband signal that is a low-frequency signal ranging from 0 Hz to several MHz, and a GPS receiver using the same.
2. Description of Related Art
Global Positioning System (GPS) navigation technology has been applied to consumer use and become widespread. Recent development of high-accuracy, high-speed positioning technology allows miniaturization of GPS receivers. This has enabled to incorporate GPS receivers into mobile terminals or the like with Ultra-Large-Scale Integration (ULSI) technology.
Following the implementation of the incorporation of GPS receivers into mobile terminals, Enhanced 911 Emergency Calling Systems (Wireless Enhanced 911 or E-911) that obligate communication common carriers to identify the calling location in the event of emergency call from mobile phones are introduced in the United States, and installation of GPS receivers into mobile phones is required. In Japan as well, as of 2007, mobile phones will be required to include GPS receivers as standard equipment. For these reasons, it is necessary not only to meet market needs such as size and cost reduction of GPS receivers but also to increase sensitivity so as to allow constantly accurate positioning.
The sensitivity of GPS receivers depends on both baseband signal processing technology and high-frequency IC technology. The baseband signal is a signal containing information, and it is a GPS signal in this specification. Presently, the baseband signal processing using Assisted-GPS (A-GPS) technology, which is called network-assisted positioning, provides a theoretical receiver sensitivity of −160 dBm. However, realization of this requires the technological development of high-frequency ICs.
There have been various configurations of high-frequency ICs for GPS receivers proposed. Normally, GPS receivers have a bandpass filter to enhance receiving selectivity. However, the bandpass limitation by bandpass filters causes loss of GPS signals, affecting the receiver sensitivity. To overcome this drawback, Japanese Unexamined Patent Publication No. 2003-240833 discloses a high-sensitivity GPS receiver that sets the bandwidth of an IF signal to 20 MHz or higher to improve the receiver sensitivity.
However, increasing the bandwidth of a bandpass filter to reduce GPS signal loss causes noise to increase. Since the performance of an automatic gain control (AGC) amplifier for amplifying GPS signals is predetermined, if the overall signal increases, the gain should be reduced accordingly. This can result in a decrease in the gain of GPS signals.
On the other hand, Japanese Examined Patent Publication No. H7-86529 discloses the GPS receiver with a low-pass filter of 450 kHz to remove the noise outside of the L1 GPS signal. However, this approach with a single low-pass filter cannot remove the noise from the signal to allow the sufficient SNR.
Therefore, it is required to increase the sensitivity for receiving GPS signals by calculating the ratio of a GPS signal to noise contained in an IF signal in detail while limiting the bandwidth of a bandpass filter to some extent.
As described above, improvement in the receiver sensitivity is demanded so as to accurately identify the position even if it is where GPS signal intensity is low such as indoors.